Final answer:
IgM binding to a pathogen initiates the classical pathway of the complement system by attaching to component C1q, leading to the pathogen being coated with C3b, which acts as an opsonin to facilitate phagocytosis.
Step-by-step explanation:
IgM on the surface of the pathogen binds to complement component C1q that initiates the classical pathway. As a consequence, the pathogen is coated with C3b, which facilitates opsonization by acting as opsonins to enhance phagocytosis. Thus, the correct answer is a) C3b; opsonins; phagocytosis.
Opsonization is the process where the pathogen is coated with molecules such as complement factors, which then assist phagocytic cells like macrophages and neutrophils in recognizing and engulfing the pathogen more efficiently. This process is integral to the immune system's ability to clear pathogens from the body.